1. Field
The present disclosure relates generally to antennas and more particularly to a steerable slot antenna comprising an array of slots in a planar waveguide.
2. Related Art
FIG. 1 shows a prior art radial line slot antenna for conversion of circular polarized waves to planar waveguide modes. See, for example, Takahashi, Takada, Ando, and Goto, IEEE Proc. H Vol 139, #1 Feb. 1992, page 77.
An upper plate 1 is provided with an array of crossed T-shaped slots 2, arranged in a spiral pattern as shown by dashed line 3. The slots 2 are designed to couple to left or right circularly polarized radiation. In particular, the spiral and slot orientations are designed to match the phase and E-vector angle of the incoming wave to the converging wave inside the waveguide.
T-configuration of each slot pair is known to suppress reflection in an optimal way, as illustrated, for example, in Hirokawa, Sakurai, Ando, Goto IEE Proc. H Vol. 137, # 6 Dec. 1990, page 367. The T-shaped slots 2 rotate along the spiral 3. The radial separation of the spiral rings of the spiral 3 is set to be one wavelength for propagation within the planar guide, after correcting for dispersion in the planar guide. As a consequence of the rotation of the T-shaped slots, the rotational phase of power coupled through the slots rotates as well. This enables the signal power within the guide to be in phase periodically in radius. In particular, the phase fronts are circular and propagate inward to a focus for the correct incident polarization.
The arrows 4 represent E-field orientation for the cylindrically symmetric TEM01 mode (with no z dependence and propagating radially inward). The circular pattern 5 represents the H field, uncorrected for phase shift from the E-field. A coaxial probe 6 in the center out-couples the vertical E-field.
A first disadvantage of this prior art is that it is a single polarization device. A second disadvantage is that the antenna does not show printing diversity. As a consequence, it is difficult, if not impossible at all, to keep the phase of the converging wavelets constant, independently of where the wave originated across the active surface of the guide.